PMMA Processing

Introduction

Polymethyl methacrylate (PMMA) was one of the first materials developed for e-beam lithography. It is the standard positive e-beam resist and remains one of the highest resolution resists available. PMMA is usually purchased in two high molecular weight forms (496 K or 950 K) in a casting solvent such as chlorobenzene or anisole. PMMA is spun onto the substrate and baked. Electron beam exposure breaks the polymer into fragments that are dissolved preferentially by a developer such as MIBK. MIBK alone is too strong a developer and removes some of the unexposed resist. Therefore, the developer is usually diluted by mixing in a weaker developer such as IPA. A mixture of 1 part MIBK to 3 parts IPA produces very high contrast but low sensitivity. By making the developer stronger, say, 1:1 MIBK:IPA, the sensitivity is improved significantly with only a small loss of contrast. A mixture of IPA:H2O (9:1) can also be used to develop PMMA, espacially for narrow patterns.

Associated documents and references

Equipment used and safety considerations

Verification prior to processing

The microfab stocks only PMMA 950K A2 and A4. They are kept in the fridge. To use PMMA, grap the bottle in the fridge and allow it to reach room temperature: about 1 hour. Then pour some of the resist into one of the amber container. Label the container and keep it at room temperature. Put the PMMA bottle back into the fridge.

Cold PMMA (or any other resist) bottle MUST NOT be opened when there are still sold as water will condensate in the bottle and deteriorate the resist quality and thus your process.

Recipe Parameters

  1. Set the hotplate at 180°C
  2. Clean your sample using solvent or acid clean procedures
  3. Dehydrate the sample on hotplate for 5 min
  4. Cool down the sample on the chill plate
  5. Spin coat PMMA with one of the Laurell programs between E and H, according to the table at the end of the page. Look-up the PMMA datasheet for more detailed information: PMMA datasheet
  6. Bake sample on hotplate at 180°C for 90 s
  7. Exposition dosage needs to be tuned according to your patterns and the resist thickness.
  8. Develop in MIBK:IPA 1:3 or IPA:H2O 9:1. The time depends on the resist thickness and the size of the patterns.
  9. Rinse in IPA after MIBK:IPA or in water after IPA:H2O for 20 s
  10. Blow dry with N2

 

PMMA A2 vs spin speed on Laurell Coater

Coating conditions are:

- spread at 500 rpm for 5 s with acceleration of 1305 rpm/s

- spin between 3000 and 5000 rpm for 45 s with acceleration of 1305 rpm/s

- bake at 180 °C for 90 s

The following table gives thicknesses vs. speed measured at the center of silicon samples of 1x1 cm².

Spin speed (rpm) Thickness measured with ellipsometer (nm) Thickness measured with reflectometer (nm)
3000 68 n/a
4000 60 n/a
5000 55 n/a